Various types of acoustic test apparatus have previously been proposed. The cross referenced earlier U.S. Pat. No. 3,872,384 discloses an apparatus to test continuity of circuits, insulation between conductors of electrical wiring, to determine voltage levels of energization in a circuit, and to provide an indication of the resistance in circuits by causing a multivibrator oscillator, powered by a local battery to oscillate at a predetermined tone, or pitch under certain conditions, the tone or pitch being modified, or the oscillations being suppressed if externally connected networks or impedances are connected to output test terminals of the apparatus.
Various types of acoustic testers have also been proposed which, usually, are limited in use, being primarily suitable to test for circuit continuity, circuit resistance, voltages, or polarity. Optically indicating testers are well known for many parameters. Optical indication, however, is frequently difficult to observe and contributes to the work load of the operator, specifically of maintenance personnel in industrial or plant establishments. In many installations it is difficult to read optical indicating pointers while simultaneously observing safety requirements or properly connecting test prongs. The positioning of optically indicating apparatus is sometimes difficult. Many of the optical indicators are comparatively restrictive in application and thus a number of them must be used for complete testing in industrial environments.